I’ve been enjoying the posts on how to interpret the PISA math scores. Everyone agrees that average math performance among American 15 year olds is disappointing with the US ranking 36th among 65 nations and subregions.

Michael Petrilli wrote a piece, PISA and Occam’s Razor, arguing that poverty might not the reason the US fares so poorly and thinks, perhaps, there’s a problem with teaching. “Maybe we’re just not very good at teaching math, especially in high school.”

There’s been an emotional, impassioned rebuttal, arguing that poverty is what is dragging the US down and we would otherwise be excellent. Do not blame the teachers.

On Diane Ravitch’s blog Daniel Wydo Disaggregates PISA Scores by Income makes the case that poverty is to blame by isolating rich schools in the US. If you looked at only schools in which fewer than 10% of the population is poor, the US would rank #1 in reading, #1 in science and #5 in math.

Of course poverty matters. The US has a real problem educating poor children. The gaps are clearly worse in high school than in elementary school. The rich-poor gap in the United States is bigger than the rich-poor gap in many other countries. But we also have a real problem with our top students.

The flaw in the Wydo analysis is that it’s silly compare students from the richest schools in America with the entire mass of another country. The PISA report clearly states that there are more variations within countries than between countries. (In Lichtenstein, a top performing country, roughly 250 points separate top and bottom students. That’s more than the point difference between top ranked Shanghai and bottom ranked Peru). Every country’s mean score is weighed down by its poor students and its unfair to compare your best students with the average student elsewhere. The fair comparison would be to compare rich schools in the United States with rich schools in Switzerland and the Netherlands. And I think you would find (I don’t immediately see the income data on the PISA website to do this number crunching) that the US would NOT compare favorably.

I think this because PISA serves up data on the 90th percentile in each nation and top American students are also below average. So poverty alone cannot explain mediocre math performance.

My conclusion is that it’s not an either or. Perhaps the US has two separate problems. One is poverty. Two is high school math education.